SIGABRT: thread lock issue in producer_avformat

Hallo mlt Devs,

first of all thanks for your hard work on MLT. I'm the mlt/kdenlive port maintainer on OpenBSD and some users send me kdenlive backtraces. It's always the same:

#0  thrkill () at -:3
#1  0x0000036f2c98665e in _libc_abort () at /usr/src/lib/libc/stdlib/abort.c:51
#2  0x0000036f2c906942 in _libc_pthread_mutex_unlock (mutexp=<optimized out>) at /usr/src/lib/libc/thread/rthread_mutex.c:265
#3  0x0000036f0f9cf372 in producer_set_up_video (self=<optimized out>, frame=0x0) at producer_avformat.c:2329
#4  producer_get_frame (producer=<optimized out>, frame=<optimized out>, index=<optimized out>) at producer_avformat.c:2960
#5  0x0000036f45687d8a in producer_get_frame (service=0x36f37b11400, frame=0x36f1e483860, index=<optimized out>) at mlt_producer.c:647
#6  0x0000036f45686c45 in mlt_service_get_frame (self=0x36f37b11400, frame=0x36f1e483860, index=<optimized out>) at mlt_service.c:593
#7  0x0000036eb9f0cc8c in Mlt::Service::get_frame (this=<optimized out>, index=0) at MltService.cpp:115
#8  0x0000036c7c002d77 in ProjectClip::doExtractImage() ()
#9  0x0000036c7bc89a39 in non-virtual thunk to QtConcurrent::RunFunctionTask<void>::run() ()
#10 0x0000036ee75d5ca7 in QThreadPoolThread::run() () from /usr/local/lib/libQt5Core.so.2.2
#11 0x0000036ee75de820 in QThreadPrivate::start(void*) () from /usr/local/lib/libQt5Core.so.2.2
#12 0x0000036edef087d1 in _rthread_start (v=<optimized out>) at /usr/src/lib/librthread/rthread.c:96
#13 0x0000036f2c97acc8 in __tfork_thread () at /usr/src/lib/libc/arch/amd64/sys/tfork_thread.S:77
#14 0x0000000000000000 in ?? ()

At first glance ,it looks like a lock problem to me because we run in the abort() call in /usr/src/lib/libc/thread/rthread_mutex.c:

	_rthread_debug(5, "%p: different owner %p (%p)\n", self, (void *)mutex,
	    (void *)mutex->owner);
		if (mutex->type == PTHREAD_MUTEX_ERRORCHECK ||
		    mutex->type == PTHREAD_MUTEX_RECURSIVE) {
			return (EPERM);
		} else {
			/*
			 * For mutex type NORMAL our undefined behavior for
			 * unlocking an unlocked mutex is to succeed without
			 * error.  All other undefined behaviors are to
			 * abort() immediately.
			 */
			if (mutex->owner == NULL &&
			    mutex->type == PTHREAD_MUTEX_NORMAL)
				return (0);
			else
				abort();

		}
	}

I would be happy if we could solve the problem together.

Start with the OpenBSD backtrace in producer_avformat.c, especially producer_set_up_video at line 2329 and producer_get_frame at line 2960, then follow the calls through mlt_producer.c. Reproduce the abort during ProjectClip::doExtractImage and inspect mutex ownership around the failing unlock. Done means identifying and fixing the thread-lock error without the SIGABRT recurring.